Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performances of antennas, RFID tags, and other wireless communication devices. An appealing approach to increase a scattering cross-section is accommodating several spectrally overlapping resonances within a structure. However, numerous fundamental and practical restrictions have been found and led to the formulation of Chu-Harrington, Geyi, and other limits, which provide an upper bound to scattering efficiencies. Here we introduce a 2D array of near-field coupled split-ring resonators and optimize its scattering performances with the aid of a genetic algorithm, operating in 19th-dimensional space. Experimental realization of the device is demonstrated to surpass the theoretical single-channel limit by a factor of >2, motivating the development of tighter bounds of scattering performances. A super-radiant criterion is suggested to compare maximal scattering cross-sections versus the single-channel dipolar limit multiplied by the number of elements within the array. This new empirical criterion, which aims on addressing performances of subwavelength arrays formed by near-field coupled elements, was found to be rather accurate in application to the superscatterer, reported here. Furthermore, the super-radiant bound was empirically verified with a Monte-Carlo simulation, collecting statistics on scattering cross sections of a large set of randomly distributed dipoles. The demonstrated flat superscatterer can find use as a passive electromagnetic beacon, making miniature airborne and terrestrial targets to be radar visible.

While plants are typically supposed to restrict the performance of radio frequency transceiver systems, they can act as efficient biogenic elements of control. A high fraction of water inside vegetation gives rise to multiple electromagnetic Mie resonances, originating from interplaying a naturally high permittivity and a form factor. Opuntia ficus-indica, known as nopal cactus, is a representative example whose succulent stems or cladodes contain nearly 75–85% water. Here, we present an Opuntia-based broadband omnidirectional antenna element, operating at several Wi-Fi communication bands, spanning from 900 MHz to 7.7 GHz. A high relative permittivity in the GHz range exceeds 20. As a result, a variety of Mie resonances within the cladode are measured and revealed by the multipole expansion technique. Modal hierarchy, resonantly excited with a coaxial cable, is demonstrated to provide a broadband impedance matching below −10 dB over the ∼150% bandwidth. Further investigations of plants as functional electromagnetic elements can contribute to the general trend of environment friendly multifunctional devices, promoting development of green technologies.

Optical bound states in the continuum (BICs) have recently attracted a great deal of attention as an efficient way to localize and manipulate light at nanoscale. Traditionally, generation of BICs has relied on using artificial structures where suppression of radiative losses leads to very high Q factors. Here, we show that BICs may play an important biological role by boosting light-matter interactions in a biogenic nanostructure: tapetum reflector of a shrimp eye. Enveloping photosensitive units of the retina (rhabdoms), this system contains quasi-periodic arrays of spherical core-shell nanoparticles which include concentric lamellae of single-crystal isoxanthopterin nanoplates arranged around a hollow core. The radial alignment of the plates gives rise to the spherical anisotropy of the nanoparticles which provides access to quasi-BIC modes in a full visible domain. Thus, a tapetum reflector hosting BICs maximizes light interactions with rhabdoms, enhancing the eye's sensitivity. Our findings suggest that BICs, previously associated with man-made structures only, can be generated in biogenic structures, performing crucial optical functionalities in living organisms.

Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performance of antennas, radio frequency identification tags, and other wireless communication devices. An appealing approach to increase a scattering cross section is accommodating several spectrally overlapping resonances within a structure. However, numerous fundamental and practical restrictions have been found and led to the formulation of Chu-Harrington, Geyi, and other limits, which provide an upper bound to scattering efficiencies. Here we introduce a two-dimensional array of near-field coupled split-ring resonators and optimize its scattering performance with the aid of a genetic algorithm operating in 19-dimensional space. Experimental realization of the device is demonstrated to surpass the theoretical single-channel limit by a factor of >2, motivating the development of tighter bounds of scattering performance. A superradiant criterion is suggested to compare maximal scattering cross sections with the single-channel dipolar limit multiplied by the number of elements within the array. This empirical criterion, which aims to address performance of subwavelength arrays formed by near-field coupled elements, is found to be rather accurate in application to the superscatterer, reported here. Furthermore, the superradiant bound is empirically verified with a Monte Carlo simulation, collecting statistics on scattering cross sections of a large set of randomly distributed dipoles. The demonstrated flat superscatterer can find use as a passive electromagnetic beacon, making miniature airborne and terrestrial targets radar visible.

Scattering cross-section is one of the main properties, characterizing an object in wireless applications. Resonant phenomena increase the electromagnetic visibility of a scatterer while keeping its footprint small. However, the single-channel limit or Chu-Harrington limit imposes a tight upper bound on a scattering cross-section of subwavelength objects. Being derived for a dipolar response, this limitation can be bypassed if several resonances of a structure are spectrally co-located and contribute constructively to the scattering. Subwavelength structures, obeying this design concept though hardly achievable in practical implementations, are called superscatterers. Here we demonstrate a superscatterer realization, based on a circular bundle of vertically aligned metal wires, optimized to demonstrate 5 multipoles, resonating at nearly the same frequency. As a result, the scattering cross-section becomes 12 times larger than object's geometrical cross-section. Owing to the multipolar multiplexing within the structure, the scattering is 7 times larger than the dipole single-channel limit. Additionally, as a result of the constructive interference of several multipoles, scattering directivity up to 10 dB is observed. Wire-bundle superscatterers may become an attractive architecture for many applications, including compact directive antennas, radar chaff and beacons, long-range RFID tags, and many others. (C) 2022 Elsevier Ltd. All rights reserved.

Functional nanocoatings of hollow-core microstructured optical fibers (HC-MOFs) have extended the domain of their applications to biosensing and photochemistry. However, novel modalities typically come with increased optical losses since a significant surface roughness of functional layers gives rise to additional light scattering, restricting the performance of functionalization. Here, the technique that enables a biocompatible and removable nanocoating of HC-MOFs with low surface roughness is presented. The initial functional film is formed by a layer-by-layer assembly of bovine serum albumin (BSA) and tannic acid (TA). The alkaline etching at pH 9 results in the reduction of surface roughness from 26 nm to 3 nm and decreases fiber optical losses by three times. The nanocoating can be fully removed within 7 min of the treatment. Natural biocompatibility of BSA alongside antibacterial and antifouling properties of TA makes the presented nanocoating promising for biophotonic applications.

Structured environments are employed in a plethora of applications to tailor dynamics of light–matter interaction processes by modifying the structure of electromagnetic fields. The promising example of such a system is antiresonant photonic crystal fibers (AR-PCFs), which allow light–analyte interactions in a very long channel. Here we probe contribution of microstructuring and nontrivial mode hierarchy on light–matter interactions in AR-PCFs by investigating lifetime shortening of perovskite ( CsPbBr 3 ) nanocrystals grown to fiber capillaries. The crystals have been deposited using a wet chemistry approach and then excited by a supercontinuum source in the 450–500 nm range. Emission spectra have been measured and analyzed via the time-correlated single photon counting (TCSPC) technique, unravelling contributions of core and cladding modes. Fluorescence lifetime imaging inside an AR-PCF enables mapping input of various electromagnetic channels into light–matter interaction processes. Our results pave the way for tailoring the dynamics of high-order quantum processes, promoting the concept of AR-PCF as a light-driven reactor.

Functional nanocoatings have allowed hollow-core microstructured optical fibers (HC-MOFs) to be introduced into biosensing and photochemistry applications. However, common film characterization tools cannot evaluate the coating performance in situ. Here we report the all-optical noncontact characterization of the HC-MOF coating in real time. Self-assembled multilayers consisting of inversely charged polyelectrolytes (PEs) are deposited on the HC-MOF core capillary, and a linear spectral shift in the position of the fiber transmission minima with increasing the film thickness is observed as small as ∼1.5-6nm per single PE bilayer. We exemplify the practical performance of our approach by registering an increase in the coating thickness from 6±1 to 11±1nm per PE bilayer with increasing ionic strength in the PE solutions from 0.15 to 0.5 M NaCl. Additionally, we show real-time monitoring of pH-induced coating dissolving. Simplicity and high sensitivity make our approach a promising tool allowing noncontact analysis of the HC-MOF coating which is still challenging for other methods.

Thermal fiber drawing is an efficient technique for producing optical fiber tapers whose operation regime gradually varies between multi-mode and single-mode. Here we present a comprehensive theoretical analysis of tapering optical fibers through changing their drawing speed on a standard fiber draw tower. We analytically calculate the drawing speed, drawing acceleration, and time needed to produce different kinds of fiber tapers and analyze the derived expressions to find the optimal fabrication parameters for the tapers. The obtained results inform how to most efficiently draw optical fiber tapers of different adiabatic shapes using the minimal pulling force for a given tapering ratio.

Mesoscopic photonic systems with tailored optical responses have great potential to open new frontiers in implantable biomedical devices. However, biocompatibility is typically a problem, as engineering of optical properties often calls for using toxic compounds and chemicals, unsuitable for in vivo applications. Here, a unique approach to biofriendly delivery of optical resonances is demonstrated. It is shown that the controllable infusion of gold nanoseeds into polycrystalline sub-micrometer vaterite spherulites gives rise to a variety of electric and magnetic Mie resonances, producing a tuneable mesoscopic optical metamaterial. The 3D reconstruction of the spherulites demonstrates the capability of controllable gold loading with volumetric filling factors exceeding 28%. Owing to the biocompatibility of the constitutive elements, "golden vaterite" paves the way to introduce designer-made Mie resonances to cutting-edge biophotonic applications. This concept is exemplified by showing efficient laser heating of gold-filled vaterite spherulites at red and near-infrared wavelengths, highly desirable in photothermal therapy, and photoacoustic tomography.

Mesoscopic photonic systems with tailored optical responses have great potential to open new frontiers in implantable biomedical devices. However, biocompatibility is typically a problem, as the flexible design of optical properties often uses toxic elements, unsuitable for in vivo applications. Here, we demonstrate a unique approach to biofriendly delivery of optical resonances. We show that the controllable infusion of gold nanoseeds into polycrystalline submicron vaterite spherulites gives rise to a variety of electric and magnetic Mie resonances and anapole states, producing a tunable mesoscopic optical metamaterial. The three-dimensional reconstruction of the spherulites shows their gold loading with volumetric filling factors of up to 28%. Owing to the biocompatibility of the constitutive elements, golden vaterite paves the way for Mie resonances to revolutionize many biophotonic applications.

Modern imaging technologies, including optoacoustic endoscopy, are based on the optoacoustic effect. Much promise is offered by the all-optical fiber-based approach, because fiber has a miniature cross section, is highly sensitive, and can be used in a variety of imaging and therapeutic techniques. We developed a probe based on a hollow-core microstructured optical waveguide (HC-MOW) with a hybrid nanostructured membrane. The membrane consisted of a free-standing single-walled carbon nanotube film and a Bragg reflector, which can be used as a source and a detector of ultrasound. Membrane vibrations were excited with an IR laser pulse and were read out by recording the intensity of the reflected visible CW laser light. We explained the nature of the intensity modulation of the reflected light and supported our explanation with numerical simulations of the membrane's vibration eigenfrequencies and thermal distribution. The membrane vibrations were also observed with raster-scanning optoacoustic mesoscopy. The transmittance of the HC-MOW between 400 nm and 6.5 mu m and that of the hybrid nanostructured membrane in the NIR range enable potential optoacoustic sensing in the IR fingerprint region of biomolecules. This permits the optoacoustic probe to be used for medical endoscopic purposes.

Abstract Hollow-core microstructured optical fibers (MOFs) possess the great potential for the integration of different materials inside the holey-capillaries leading to the creation of tailored hybrid structures. Moreover, the further improvement of MOF-based sensor performance can be achieved by exploiting the wide range of post-processing techniques directing to both the enhancement of the existing characteristics and the enabling of new functionalities. Here, we concentrate on hybrid MOFs whose hollow-capillaries were coated through the layer-by-layer assembly technique by a combination of oppositely charged polyelectrolytes and magnetite nanoparticles. We characterize the optical transmission and the fiber loss of the modified samples and show the scanning electron microscopy images illustrating the formed coatings on the inner fiber surfaces.

Vaterite is a very promising material for biological applications, but its electromagnetic properties have not been studied well enough. In this work we for the first time introduce multipole decompositions of subwavelength nanoparticles for dark field microscopy in optical range.

The concept of lumped optical nanoelements (or metactronics), wherein nanometer-scale structures act as nanoinductors, nanocapacitors, and nanoresistors, has attracted a great deal of attention as a simple toolbox for engineering different nanophotonic devices in analogy with microelectronics. While recent studies of the topic have been predominantly focused on linear functionalities, nonlinear dynamics in microelectronic devices plays a crucial role and provides a majority of functions, employed in modern applications. Here, the metactronics paradigm is extended and nonlinear dynamical modalities are added to those nanophotonic devices that have never been associated with optical nanoantennas. Specifically, it is shown that nonlinear dimer nanoantennae can operate in the regimes of tristable and astable multivibrators as well as chaos generators. The physical mechanism behind these modalities relies on the Kerr-type nonlinearity of nanoparticles in the dimer enhanced by a dipolar localized surface plasmon resonance. This allows one to provide a positive nonlinear feedback at moderate optical intensities, leading to the desired dynamical behavior via tuning the driving field parameters. The findings shed light on a novel class of nonlinear nanophotonic devices with a tunable nonlinear dynamical response.

Interference phenomena allow tailoring propagation of electromagnetic waves by controlling phases of several scattering channels. Huygens element, being a representative example of this approach, enables enhancement of the scattering from an object in a forward direction, while the reflection is suppressed. However, a typical resonant realization of Huygens element employs constructive interference between electric and magnetic dipolar resonances that makes it relatively narrowband. Here we develop the concept of a broadband resonant Huygens element, based on a circular array of vertically aligned near-field coupled metal wires. Accurate management of multipole interference in an electrically small structure results in directional scattering over a large bandwidth, acceding 10% of the carrier frequency. Being constructed from nonmagnetic materials, this structure demonstrates a strong magnetic response appearing in dominating magnetic multipoles over electric counterparts. Moreover, we predict and observe higher-order magnetic multipoles, including hexadecapole (M16-pole) and magnetic triakontadipole (M32-pole) with quality factors, approaching 6000. The experimental demonstration is performed at the low GHz spectral range. Broadband Huygens elements can be employed in a set of practical applications, where compact electromagnetic devices for tailoring wave propagation are needed, i.e., antenna devices, directional reflectors, and even solar cells, given that the concept is scaled to the optical frequency range.

The state of the art in optical biosensing is focused on reaching high sensitivity at a single wavelength by using any type of optical resonance. This common strategy, however, disregards the promising possibility of simultaneous measurements of a bioanalyte’s refractive index over a broadband spectral domain. Here, we address this issue by introducing the approach of in-fibre multispectral optical sensing (IMOS). The operating principle relies on detecting changes in the transmission of a hollow-core microstructured optical fibre when a bioanalyte is streamed through it via liquid cells. IMOS offers a unique opportunity to measure the refractive index at 42 wavelengths, with a sensitivity up to ~3000 nm per refractive index unit (RIU) and a figure of merit reaching 99 RIU −1 in the visible and near-infra-red spectral ranges. We apply this technique to determine the concentration and refractive index dispersion for bovine serum albumin and show that the accuracy meets clinical needs.

In article number 1900304, Pujuan Ma, Lei Gao, Pavel Ginzburg, and Roman E. Noskov reveal that nonlinear graphene-coated dimer nanoantennae can scatter continuous-wave radiation in the dynamic regimes associated with the functionalities of tristable and astable multivibrators, as well as chaos generators. The findings point to a novel class of nonlinear nanophotonic devices with a tunable dynamic response.

Microstructured optical waveguides (MOW) are of great interest for chemical and biological sensing. Due to the high overlap between a guiding light mode and an analyte filling of one or several fiber capillaries, such systems are able to provide strong sensitivity with respect to variations in the refractive index and the thickness of filling materials. Here, we introduce a novel type of functionalized MOWs whose capillaries are coated by a layer-by-layer (LBL) approach, enabling the alternate deposition of silica particles (SiO2) at different diameters—300 nm, 420 nm, and 900 nm—and layers of poly(diallyldimethylammonium chloride) (PDDA). We demonstrate up to three covering bilayers consisting of 300-nm silica particles. Modifications in the MOW transmission spectrum induced by coating are measured and analyzed. The proposed technique of MOW functionalization allows one to reach novel sensing capabilities, including an increase in the effective sensing area and the provision of a convenient scaffold for the attachment of long molecules such as proteins.